Download TSM53 - The External, Middle, and Inner Ear

TSM53: THE EXTERNAL, MIDDLE AND INNER EAR
28/10/08
LEARNING OUTCOMES
Describe the structure of the external ear, the middle ear and the inner ear
EXTERNAL EAR
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The external ear consists of the auricle and external auditory meatus
The auricle or pinna is a mostly immobile cartilaginous ‘flap’ that aids sound localisation
o Outer helix and inner antihelix – cartilaginous ridges
o Laterally projecting crura of the antihelix superiorly and of the helix inferiorly
o Anterior tragus and infero-posterior antitragus – cartilaginous prominences
o Inferior lobule (‘ear lobe’) continuous with helix – soft fatty projection
The external auditory meatus or canal is a narrow passage leading to the tympanum
o Lateral third has cartilaginous walls, medial two thirds are osseous within the temporal bone
MIDDLE EAR – INTERNAL STRUCTURE
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The middle ear consists of the tympanic cavity and ossicles
The tympanic membrane is effectively the border between the external and middle ear
o Lateral surface of the membrane faces infero-anteriorly
o Superior prominence made by the lateral process of malleus
o Internal folds on either side of the lateral process – anterior and posterior malleolar folds
 Superior to these folds the membrane is thin and non-fibrous – pars flaccida
 Inferior to the folds it is thick and tense – pars tensa
o Internal inferior projection from the lateral process – handle of malleus
 Attaches centrally to the pars tensa at the umbo
When using an otoscope to view a healthy tympanic membrane an antero-inferior light reflection can
be seen – the cone of light – which is useful for orientation when viewing otoscopic images
The three ossicles link together within the tympanic cavity to transmit sound to the inner ear
o The malleus is ‘T’ shaped and articulates with the incus superiorly at its head
o The incus is ‘L’ shaped, articulates with the malleus at its base and the stapes at its long limb
o The stapes is ‘D’ shaped, articulates with the incus at its head and the oval window at its base
MIDDLE EAR – WALLS
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There are four principal openings in the walls of the middle ear:
o Labyrinthine (medial) wall – border of the inner ear
 Oval window – entrance to the vestibule, sealed by the footplate of stapes
 Round window – entrance to the scala tympani, sealed by mixed fibrous membrane
o Anterior wall
 Eustachian tube – to the nasopharynx, facilitating pressure equalisation
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Mastoid (posterior) wall
 Aditus to mastoid antrum – to the air cells of the mastoid process
INNER EAR – OVERVIEW
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The inner ear consists of the vestibule, semicircular canals and cochlea within a bony labyrinth
o This bony labyrinth is part of the petrous temporal bone
o Unlike the external and middle ears, all parts of the inner ear are fluid filled
 The bony labyrinth itself is filled with clear perilymph
 Within the bony labyrinth is a membranous labyrinth filled with endolymph
The membranous labyrinth comprises two ducts and two sacs:
o Utricle and saccule – within the vestibule, part of the vestibular system
o Semicircular ducts – within the semicircular canals, part of the vestibular system
o Cochlear duct – the passage within the cochlear through which sound is transmitted
Describe the muscles of the middle ear
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There are two muscles associated with the middle ear:
o Tensor tympani – tenses the tympanum in response to loud sounds – reduces vibrations
 Runs above and alongside the auditory tube
 Inserts onto upper handle of malleus behind the tympanum
 Innervated by a branch of trigeminal V3 (mandibular nerve)
o Stapedius – displaces the stapes posteriorly in response to loud sounds – reduces oscillation
 Arises from the pyramidal eminence in the mastoid (posterior) wall
 Inserts onto the neck of stapes
 Innervated by a branch of the facial nerve (CNVII)
Describe the functions of the external and middle ears
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Sound striking the pinna is eventually recognised by the brain as external sound
o Inner-ear headphones effectively bypass the pinna and are perceived as ‘internal sound’
o The external ear also aids sound localisation in the vertical plane
The external auditory meatus protects components of the middle and inner ear from damage
o Dust, foreign bodies and so on are captured in a waxy secretion – cerumen
The middle ear acts as a transformer between the air-filled external ear and fluid-filled inner ear
o Sound waves passing directly from air to fluid are mostly reflected
o Since fluid is much denser than air it has a far higher acoustic impedance
o Matching input impedance to output impedance minimises reflection
Specific acoustic impedance (z) of a medium involving a single wave frequency is:
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where p is sound pressure, v is particle velocity
The middle ear concentrates sound pressure and reduces particle velocity thus increasing the ‘input
impedance’ into the fluid-filled inner ear to minimise reflection
o The surface area of the tympanum is roughly 18 times that of the footplate of stapes
o The intermediate ossicles also act as levers which amplify the force by a factor of 1.3
o The ratio of force to surface area corresponding to pressure in total increases 25-fold
Describe the role of the inner ear in sound transduction
INNER EAR – COCHLEA
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The cochlea is a stacked bony spiral with two to three ‘levels’ containing the organs of hearing
o The central bony column of the cochlea is called the modiolus
o The cochlear nerve (branch of CNVIII) ascends the core of the modiolus
o A bony shelf, the spiral lamina, projects laterally from the modiolus throughout the cochlea
o The endolymph-filled cochlear duct encircles the modiolus and attaches to the spiral lamina
o Above the cochlear duct runs another canal, the scala vestibuli, continuous with the vestibule
o Below the cochlear duct runs the scala tympani, terminating at the round window
o Point of connection between the scala vestibuli and tympani at the apex, the helicotrema
The cochlear duct is triangular with its base facing laterally and apex medially towards the modiolus
o Its outer wall (spiral ligament) has a thickened epithelial layer, the stria vascularis
o Its roof has a thin membrane separating it from the scala vestibuli, Reissner’s membrane
o Its floor has a thicker membrane separating it from the scala tympani, the basilar membrane
The organ of hearing, the organ of Corti, is attached to the basilar membrane
o Has an overlying tectorial membrane which interacts with stereocillia of underlying hair cells
 Three or four layers of outer hair cells connect directly to the membrane
 Single layer of inner hair cells does not connect directly
o Cochlear nerve fibres are stimulated by neurotransmitter released by the inner hair cells only
INNER EAR – SOUND TRANSDUCTION
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Pressure waves entering the inner ear via the oval window propagate through the vestibule
o These waves ascend the scala vestibuli to the helicotrema at the apex of the cochlea
o They then descend the scala tympani which terminates at the round window
As sound waves ascend the scala vestibuli they cause the basilar membrane to vibrate and displace
o Displacement of the basilar membrane excites the hair cells
o High frequencies produce maximal displacement at the base of the cochlea
o Low frequencies produce maximal displacement at the apex of the cochlea
Distortion of the stereocillia of the hair cells moderates membrane potassium channels
o Bending ‘outward’ triggers potassium influx and depolarisation
o Bending ‘inward’ reduces potassium influx and promotes hyperpolarisation
Depolarisation of the inner hair cells causes the release of an excitatory neurotransmitter
o This stimulates nearby cochlear nerve fibres and transmits information to the CNS
The outer hair cells are essentially highly sensitive modifiers and amplifiers
o Enable our hearing to be as sensitive as it is
o Damaged by some antibiotics, particularly aminoglycosides (broad spectrum)